Need advice on a spouse that wants a baby between us, when I brought 3 with me and we have 1 we adopted.?

Here are the details. She has no children of her own, but has been married twice. I have 3 biologically and share custody with ex-spouse, but 2 boys stay with me. Prior to marriage new spouse understood how I felt and that I had had a vasectomy. So, we compromised and adopted a 3 year old a year ago. Now she says the feelings have returned and the now 4 year old is interested in a younger sibling. My spouse is 34 and I am 40 and she really wants a baby of her own. I can understand the feelings and urges. She is "thinking" about it which means if she can not deal with the feelings, we will divorce so she can find a sperm donor or new husband so that she can have a bio-baby. My concern is for the ex-tented family and for our adopted daughter. I can not make up her mind for her and yes I want her happy, but seems to me she is being a little selfish and NOT seeing the big picture. I mean she did agree to marry me with a V and agree to an adopted daughter. Now after nearly 4 years, she is willing to toss it away because I do not want a 5th child, first for her biologically. We are fine financially, but I do not want any more babies. I told her I am even willing to adopt again, just not a baby. I did that 3 times and got a v, so no more for me. No more crying, diapers, late nights, etc. Help. Thanks in advance.